The present invention relates to an electronic circuit for monitoring electrical voltage, and more particularly to such a circuit having a high threshold terminal and a low threshold terminal, and means for comparing the voltage sensed on these terminals with at least one reference voltage. A reference voltage is used to determine whether the voltage on the high threshold terminal is not greater than a predetermined threshold and whether the voltage on the low threshold terminal is not less than a predetermined threshold, which can either be equal to or different from the predetermined threshold for the high threshold terminal.
Such circuits, of the window-based comparator type, are already known. The difficulty arises during fault detection, when it is impossible to determine if the fault stems from the voltage monitored or from the monitoring circuit itself. The monitoring circuit may in fact signal a fault stemming from its own deficiency, when the monitored voltage is normal.
Conversely, and this is the essential point, the monitoring circuit may be faulty, and may fail to signal a fault of the circuit generating the voltage to be monitored. This is important, particularly in the field of avionics, where a fault can sometimes be tolerated because the faulty equipment is backed-up, (i.e., two or three times). It is still important to know that the fault exists.
To this end, according to a first aspect of the invention, there is a monitoring circuit of the type recited above, characterized in that it includes means for prescribing the voltage on the high threshold terminal at a high value greater than a predetermined threshold, and means for prescribing the voltage on the low threshold terminal at a low value less than a predetermined threshold. The low value predetermined threshold may be different than the high value predetermined threshold.
These means make it possible to switch to self-test mode. Indeed, if it is operating normally, the monitoring circuit will have to signal a fault, since the voltage on its high threshold terminal will be less than the predetermined threshold and since, likewise, the voltage on its low threshold terminal will be greater than the predetermined threshold.
It is also evident that a monitoring circuit of the above type may be used to read a Boolean level on each of its inputs, which in this case are independent. Here again this involves providing means allowing a self-test.
To this end, according to another aspect of the invention, there is a monitoring circuit of the type recited above, characterized in that it includes means for prescribing the voltage on the high threshold terminal at a high value greater than a predetermined threshold, and means for prescribing the voltage on the low threshold terminal at a low value less than a predetermined threshold. The low value predetermined threshold may be different than the high value predetermined threshold.
More particularly, the circuit according to the invention may include two comparators, one of the comparators configured to receive on a first of its inputs, the high threshold terminal, during voltage level monitoring, a high voltage derived from the voltage to be monitored, and having a first reference voltage on its other input, the other comparator configured to receive on a first of its inputs, the low threshold terminal, during voltage level monitoring, a low voltage derived from the voltage to be monitored and a second reference voltage on its other input; a first switch provided to apply a high voltage greater than a first reference voltage to one or the other of these inputs, and a second switch making it possible to apply to the other input a low voltage less than the second reference voltage. The low value predetermined threshold may be different than the high value predetermined threshold.